The production of high-precision castings requires stable support for casting cores within a casting mold. In the course of the casting operation, the so-called chaplets required for this leave openings in the walls of the casting, which in most applications represent undesired weakening points of the casting, but in particular also undesired leakage points. In this connection, reference should be made in particular to the production of cooled turbine blades, in the interior of which complexly configured air channels are fashioned and these channels have to be formed in a gastight manner in the turbine blade to avoid undesired coolant losses. To produce the internal structure of such blades, casting cores have to be fixed in a very precise and stable manner in the casting mold, fixing the casting cores, which are oriented essentially in the longitudinal direction of the blade, on two sides, i.e. on the side of the blade root and on the side of the blade head, by means of chaplets of relatively large dimensions, which in turn result in large core openings in the casting once casting has been completed, but these are not all desired for proper functioning of the casting. For example, in the case of a cooled gas turbine blade, openings of relatively large dimensions are desired at the blade root in order to feed cooling air into the interior of the blade or remove it again, but particularly openings at the blade head or end of the airfoil of the blade represent undesired openings through which harmful cooling air leakages occur. Subsequent closing of such core openings caused by the casting technique, particularly in the end region of the airfoil of the blade, requires great care and consideration for the operating conditions to which the blade is exposed. For instance, the closure must be made resistant to heating and to temperature changes and stable enough with respect to the centrifugal forces occurring in the case of moving blades.
On the one hand, it would be desirable with respect to production considerations to use chaplets that are as large as possible, which however leads to large core openings also occurring in undesired regions of the casting, but on the other hand there is the need for these very core openings of large dimensions to be reliably closed. The closure mechanisms that have become known in the relevant literature are problematic when used on turbine blades, which undergo high thermal loads: for example, it is proposed in DE 39 36 171 A 1 to close core openings by means of buildup welding. Here, however, there is the latent risk of the placed-on closure piece, which is connected to the casting by a welded connection, becoming detached from the casting and exposing the previously closed opening again. Resultant consequential damage, in particular in connection with moving blades closed in such a way in rotor arrangements of gas turbine installations, is considerable.
In the documents U.S. Pat. No. 2,821,323 and DE 44 34 139 C1, closure plugs driven axially into the core opening are proposed for closing the core openings, but the associated risk of detachment of the corresponding closure plugs caused by centrifugal force, possibly additionally assisted by differential thermal expansions occurring between the closure plug and the casting, cannot be eliminated.
To avoid possible operationally caused detachment of a closure plug referred to above from the core opening of a casting, preferably a rotating gas turbine blade, it is proposed in DE 199 05 887 C1 to introduce a closure piece that closes the core opening in the casting along a clearance which extends transversely in relation to the core opening to be closed within the casting. However, apart from the already existing core opening within the casting, such a measure requires further local removal of material from said casting, causing it to be mechanically weakened further.